11. South Corridor Option 4A
South Corridor Option 4A is a combination of South Corridor Options 1 and 2, providing connectivity to the proposed replacement bridge from the A90 and the M9. An indicative layout of this option is provided in Appendix 9.
The connection to the A90 would be used to facilitate direct access to the north of Edinburgh from the proposed replacement bridge, relieving some of the traffic pressures which may build up on the existing road network through the implementation of South Corridor Option 2 in isolation.
Operating as a single motorway on approach to the proposed replacement bridge, a divergence of carriageways is required southwest of South Queensferry. A new junction would be provided to the A904 from the South Corridor Option 1 mainline, maintaining cross-Forth connectivity and access to Edinburgh and the central Scotland motorway network for local communities.
In the provision of access to the M9 through the South Corridor Option 2, a new junction would be provided encompassing the existing M9 Junction 1a.
Through the implementation of this option, benefits to road based cross-Forth public transport may also be realised, connection to the A90 providing a direct link to central Edinburgh via Barnton Junction and Queensferry Road (A90).
No consideration is made for HOV traffic with South Corridor Option 4A.
The provision of South Corridor Option 1 and South Corridor Option 2 in combination would impact the oil pipeline, with multiple crossings required within the grounds of Dundas Home Farm and Dundas Estate.
Due to the increased mainline functionality offered, a carriageway cross section to D2M standard would be implemented on both corridors.
11.2 Engineering Issues
A number of engineering constraints were identified which require assessment to allow further development should this option be progressed. Principally these are:
- Route Corridor, Structural Requirements and Topographical Effects
- Geotechnical Assessment
This section shall focus on Junction Provision and Future Transport Developments.
11.2.1 Junction Provision and Future Transport Developments
In the provision of South Corridor Option 4A, new junctions are required to the M9 and the A904. To facilitate access between the two corridors, as a minimum, simple north facing connections are also required. The reconstruction of Scotstoun Junction can also be considered in the provision of this option.
(a) M9 Junction
The provision of a new junction on the M9 is required to maintain cross-Forth links between central Scotland and the north. Full connectivity would be provided between the M9 and the South Corridor Option 4A (South Corridor Option 2) mainline. Unlike South Corridor Option 2, the junction provided need not encompass the existing M9 Junction 1a. However, given the operational concerns associated with M9 Junction 1a, improvements to the existing layout can be incorporated, the new arrangement being capable of providing west facing connectivity from the M9 Spur to the M9 and an alternative connection from the M9 to the M9 Spur, replacing the existing loop arrangement.
(b) South Corridor Option 1 – South Corridor Option 2 Connectivity
The connection between South Corridor Option 1 and South Corridor Option 2 is made in proximity to the A904, providing route choice to traffic departing the proposed replacement bridge. The South Corridor Option 2 mainline forms the priority route to the proposed replacement bridge. The junction connecting the corridors may take the form of the following:
- A simple north facing slip road arrangement providing access to and from the proposed replacement bridge.
- A grade separated junction arrangement providing northbound access to the proposed replacement bridge, southbound access to the South Corridor Option 2 mainline and limited access to the A904.
(c) Echline Junction
In the provision of non-motorway connectivity and to maintain local access to the proposed replacement bridge, a new junction is required to the A904 west of South Queensferry.
Connectivity to the Forth Road Bridge must also be considered in the development of this junction, thus catering for any future role for which the existing bridge might have.
In the provision of South Corridor Option 4A, the following proposals could be implemented:
- a grade separated junction utilising a roundabout to maintain all movements between the proposed mainline and the A904.
- a free flow junction arrangement, incorporating improvements to Scotstoun Junction and the use of the existing Echline Junction, providing access between the proposed mainline, the A904 and the A8000.
The development of either arrangement shall be the subject of further traffic analysis should South Corridor Option 4A be taken forward to DMRB Stage 2 Corridor Assessment.
(d) Scotstoun Junction
In the provision of South Corridor Option 4A, Scotstoun could be utilised in its current form, providing limited functionality between the M9 Spur and A90 or upgraded to provide full connectivity.
If reconstruction was to be considered, the new free flow, all movements junction would provide the following:
- a priority routing of traffic between the M9 Spur and the A90 to the east
- a new slip road arrangements between the M9 Spur and the A90 to the west
- new slip road arrangements maintaining A90 through traffic connectivity between Edinburgh and local destinations via the A904/A8000.
11.3 Environmental Considerations
Land take for Route Corridor Option 4A is high; however, the land use impacts are not expected to be as great as for Options 2 or 3 which have higher impacts in relation to agricultural land or property demolitions.
South Corridor Option 4A is considered to have the greatest potential ecological impacts due to habitat loss and fragmentation. Similar to South Corridor Option 2, it would be likely to have the greatest impact on the water environment with a high number of watercourse crossings required.
11.4 Transportation Considerations
As a combination of South Corridor Options 1 and 2, through the provision of a D2M cross section in tandem with new junctions to the M9 and A904 and the possible reconstruction of Scotstoun Junction is expected to provide the following benefits:
- improved existing levels of service for private, road-based modes of travel
- improved network performance.
- improved journey time reliability through the provision of new route corridor options for some journeys between central Scotland and Fife.
- minimum change to land-based travel choices and integration.
- improved general accessibility for those with access to private transport.
- minimal impact on the effective operation of the transport network during times of maintenance.
- reduced sustainable development, but increase economic growth.
11.5 South Corridor Option 4A Recommendation
An indicative layout of South Corridor Option 4A is contained within Appendix 9. If carried forward to DMRB Stage 2 Corridor Assessment this route corridor option could encompass the following:
- a dual two lane motorway (D2M) mainline cross section;
- a new M9 Junction providing partial connectivity between the M9 and the M9 Spur and full connectivity between the M9 and the South Corridor Option 4A mainline; and
- a new junction to the A904 providing full connectivity.
In addition, the following junction improvements could also be considered:
- the reconstruction of Scotstoun Junction